1. Field of the Invention
This invention is directed to a machine for accurately metering two or more liquids, which are reactive to rapidly form a polymer, in precise stoichiometric proportions and then intimately and thoroughly admixing the liquids to initiate the polymer forming reaction, and injecting the reacting mixture into a mold.
Recently, polymer chemists have developed polymerization systems that proceed rapidly to completion in the bulk state, i.e., less than one minute. The most important of these are the fast urethane polymerizations, but other systems such as silicone rubber, nylon and polyesters can also be rapidly polymerized in bulk. In order to polymerize completely to high molecular weight, the monomers or pre-polymers involved must be rapidly and completely mixed in precisely correct stoichiometric proportions.
The potential advantages of forming objects by injection molding using such fast polymerizing systems are great. The mold can be filled rapidly and at low pressure because the monomers involved are of low viscosity. The final polymer forms in the mold. Great savings in capital, equipment and in energy can be realized from the use of equipment to automatically meter, mix and inject the reactive liquids. Heavy molds, high clamp force, and large injection units are no longer necessary.
2. The Prior Art
The use of fast polymerizing systems in injection molding is called "reaction injection molding" or "RIM". All practical RIM machines use impingement mixing in which two or more reactant streams collide in a chamber. To prevent plugging, the chamber is cylindrical and wiped clean after each shot with a closely fitting ram. The flow rate through the mixing head must be high to get good mixing. Such flow rates are generally sufficient to fill molds rapidly with the low viscosity reactants. Mixing units with moving impellers or complex internal structures are generally undesirable because they tend to become plugged with polymer due to the fast reaction.
The principal differences in prior RIM machines is in the metering system and to some extent the type of mix head. A continuous pump type of machine uses positive displacement piston pumps to meter reactants. Such machines are expensive, present serious maintenance problems, and are prone to failure. Because the pumps do not respond rapidly enough, they may deliver materials off-ratio at the beginning of the shot.
Other machines use two hydraulic cylinders to meter the reactive components. These are mechanically simpler than pump type machines but require some means of altering and then maintaining stoichiometric ratio between the two cylinders. One method is to have the cylinders facing in opposite directions and drive an adjustable wedge between them. This simple approach suffers from long connecting lines and large bending moments which can jam the pistons of the cylinders and wear the seals. Another approach is to control metering with a sophisticated servo feed back system on two power cylinders. This equipment is also expensive.
Another machine employs an accumulator system and gear pumps but does not appear to be capable of accurate metering.
A variety of mix heads have also been proposed. One type of machine uses a complex impingement mix head patterned after a diesel engine fuel injector, similar to that of Fries U.S. Pat. No. 3,936,036. Another head uses slits for the impinging streams rather than circular orifices. The slit design allows the wiper to uncover any portion of the slit permitting easy changing of fluid velocity. See, for example, Wisbey U.S. Pat. No. 4,043,486.